Use of reaction mixtures containing polyester polyols in the production of solid polyurethane materials by casting

ABSTRACT

The invention relates to the use of reaction mixtures containing polyester polyols which have been obtained by ring opening of epoxidized esters with carboxylic acids, as polyol component, optionally in admixture with polybutadiene diol, in the production of solid polyurethane materials by casting.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is concerned with polyurethane materials andrelates to the use of reaction mixtures containing polyester polyols aspolyol component in the production of solid polyurethane materials bycasting.

Solid polyurethane materials produced by casting are widely known in theprior art. They generally consist of the so-called "casting resin", acompound containing at least two isocyanate group and a polyol component("hardener"). Mixing of the polyol component with the casting resincontaining isocyanate groups produces a reaction mixture which,optionally after the introduction of desired additives, is cast into amold where it is able to cure during the polyaddition reaction to formpolyurethane polymers. A review of polyurethane casting systems can befound, for example, in E. W. Becker and D. Braun "Kunststoffhandbuch",Vol. 7, Polyurethanes, Verlag-Hanser, 1983 or in B. A. Dombrow,"Polyurethanes", 2nd Edition, New York (1965).

Above all, there is a need for polyol components which may be derivedfrom renewable raw materials. Some time ago, the natural product, castoroil, was used as hydroxyl component in the production of polyurethanematerials ("Plastics Laboratory"--Princetown University, "Castor OilPolyurethanes and Applications as Potting Components", U.S.O.T., pages99 to 165). The low viscosity of castor oil proved to be an advantage byproviding for easy processing during the production of the polyurethanematerials. Polyurethane materials obtained using castor oil now nolonger satisfy present-day requirements in regard to tensile strength,hardness, resistance to thermal deformation and electrical properties.

2. Discussion of Related Art

Accordingly, it is proposed in DE-OS 36 30 264 and European patentapplication 0 125 579 to use ring-opening products of epoxidizedtriglyceride oils with monohydric lower alcohols instead of castor oilas the polyol component. However, polyurethane materials based onpolyether polyols show a greater tendency towards inflammability and apronounced tendency towards hydrophilicity. In particular, the tendencytowards hydrophilicity leads to increased bubble formation during theprocessing and/or storage of the polyurethane materials in the presenceof atmospheric moisture, so that product quality is reduced.

Improved flameproof behavior and greater stability to hydrolysis isobserved in the case of polyurethane materials based on polyesterpolyols.

U.S. Pat. Nos. 2,882,249 and 4,025,477 relate to polyurethane materialsof which the polyester polyol component has been produced byring-opening of epoxidized soybean oil with ricinoleic acid (U.S. Pat.No. 2,882,249) or with acrylic acid (U.S. Pat. No. 4,025,477). In bothpatent specifications, the polyurethane materials are produced byreaction of the isocyanate component with the polyester polyols eitherwith heating or in the presence of an organic solvent. However,relatively high temperatures are undesirable in the production ofpolyurethane materials because polyurethanes have the characteristic ofreversion i.e. back reaction of the urethane bonds to the startingmaterials. In addition, the use of organic solvents is both ecologicallyand economically questionable and should therefore be avoided.

In the production of polyurethane materials, the problem of inadequatecompatibility of the isocyanate component with the polyol componentoften arises, so that unwanted separation and clouding can occur in thefinal polyurethane material. In addition, the production of polyurethanematerials requires starting materials of low viscosity so thatproduction and processing can be carried out in the absence of bubbleformation.

Accordingly, the problem addressed by the present invention was toenable isocyanate-compatible polyester polyols of low viscosity to beused as polyol component in the production of polyurethane materials bycasting. Using polyester polyols such as these, polyurethane materialswould even be able to be produced without having to work at elevatedtemperatures or in the presence of an organic solvent. In addition,polyurethane materials obtained using the polyester polyols in questionwould be of very low viscosity and would be substantially inflammableand would also show improved properties in relation to polyurethanematerials obtained from castor oil.

Accordingly, the present invention relates to the use of reactionmixtures containing polyester polyols, which can be cast at roomtemperature and which have been obtained by at least partial ringopening of epoxidized esters with carboxylic acids, as polyol componentin the production of polyurethane materials by casting.

In the context of the invention, reaction mixtures containing polyesterpolyols are understood to be ring-opening products of epoxidized esterswith carboxylic acids which, on a statistical average, contain more than1 and preferably more than 1.5 free hydroxyl groups and ester groups permolecule.

According to the invention, it is possible to use esters of epoxidizedacids and/or epoxidized alcohols. The production of an ester containingan epoxidized alcoholic component is normally carried out by epoxidationof the unsaturated alcohol after esterification thereof with acarboxylic acid. The production of an ester containing an epoxidizedacid component is normally carried out by epoxidation of an unsaturatedcarboxylic acid after esterification thereof with an alcohol. Theproduction of an ester containing both an epoxidized alcohol componentand an epoxidized acid component is carried out by a combination of theabove methods. The epoxidation is carried out by methods known per se,for example by complete or substantially complete epoxidation by theprocess according to DE-PS 10 42 565. Partial epoxidation is alsopossible when a polyunsaturated compound is present, although in thatcase the epoxidized esters formed must contain on a statistical averagemore than one epoxide group per molecule.

It is preferred to use esters of epoxidized carboxylic acids withmonohydric to trihydric alcohols containing up to 40 carbon atoms,preferably up to 36 carbon atoms and, more preferably, 1 to 22 carbonatoms. Particularly preferred esters are triglycerides with epoxidizedcarboxylic acids and, more particularly, with epoxidized fatty acidscontaining up to 40 carbon atoms, preferably up to 36 carbon atoms and,more preferably, up to 22 carbon atoms. Suitable starting materials forthe preferred epoxidized triglycerides are any of the numerous animaland/or vegetable triglycerides with mono- and/or polyunsaturated fattyacid residues, such as palm oil, lard, ground nut oil, rapeseed oil,cottonseed oil, soybean oil, train oil, sunflower oil, coriander oiland/or linseed oil. In one preferred embodiment of the invention,epoxidized soybean oil (epoxide oxygen content 5.8 to 6.5% by weight),epoxidized sunflower oil of high and/or low oleic acid content (epoxideoxygen content 4.4 to 6.6% by weight), epoxidized linseed oil (epoxideoxygen content 8.2 to 8.6% by weight) and epoxidized train oil (epoxideoxygen content 6.3 to 6.7% by weight) are used as the epoxidized esters.

According to the invention, the epoxidized reaction components arecompletely or at least substantially completely ring opened by delayedaddition to carboxylic acids introduced beforehand, preferably inaccordance with earlier German patent application P 39 35 127.0.According to the invention, monocarboxylic acids are initiallyintroduced for the ring opening reaction. Suitable monocarboxylic acidsare synthetic, natural, aliphatic, aromatic, araliphatic and/or cycliccarboxylic acids or mixtures thereof. Monocarboxylic acids or mixturesof saturated, unsaturated, branched and/or unbranched carboxylic acidscontaining up to 24 carbon atoms and preferably 6 to 18 carbon atoms arepreferably used, caprylic acid, capric acid, behenic acid, palmitoleicacid, oleic acid, linoleic acid and/or linolenic acid being particularlypreferred.

The epoxidized reaction components are introduced with delay intocarboxylic acids introduced beforehand, preferably in quantities of upto 1:10, more preferably in at most equimolar quantities and, mostpreferably, in slightly less than equimolar quantities, expressed asmol-% epoxide and based on mol-% acid groups. According to theinvention, the delayed addition of the epoxidized reaction component iscarried out in such a way as to avoid substantial proportions ofunreacted epoxide groups in the reaction mixture. Quantitativedetermination of the substantial proportions in the process according tothe invention is carried out by comparison with the unreacted epoxidegroups by the so-called one-pot process. In the one-pot process, theepoxidized reaction components are completely added all at once to thecarboxylic acid introduced beforehand. Under the same reactionconditions, i.e. the same type and quantities of epoxidized reactioncomponents and carboxylic acids initially introduced, the content ofunreacted epoxide groups in the reaction mixture after addition of thetotal quantity of epoxidized reaction component is lower by up to 50%,expressed as the epoxide oxygen content in % by weight and based on thereaction mixture as a whole, in the dropwise addition process accordingto the invention than in the one-pot process. Distinctly lower contentscan be obtained if the addition rate is kept very low. For economicreasons, however, the addition should be made as quickly as possible andpreferably in 1.5 hours or less. The content of unreacted epoxide groupsmay be determined by titration using R. R. Jay's method (AnalytischeChemie 36 (1964), pages 667 et am).

According to the invention, the epoxidized reaction components are addedto carboxylic acids introduced beforehand, the carboxylic acids havingbeen heated to temperatures above 80° C. and below 300° C. andpreferably to temperatures above 100° C. and below 270° C. The actualreaction temperature is determined above all by the carboxylic acidsintroduced beforehand. Thus, reaction temperatures in the lower rangeare preferred with lower carboxylic acids while reaction temperatures inthe upper range are preferred for higher carboxylic acids to ensure thatthe carboxylic acids are present in the liquid state. The reactionmixture is left to react at the above-mentioned reaction temperaturesuntil an absolute residual epoxide oxygen content in the reactionmixture of less than 1.0% by weight, preferably less than 0.5% by weightand, more preferably, less than 0.3% by weight is reached.

According to the invention, the excess carboxylic acids, if any, areremoved from the reaction mixture after the epoxide groups have reactedoff to the desired residual epoxide content. The excess carboxylic acidsare preferably removed by distillation in vacuo, although they may alsobe removed by other methods of separation, such as neutralization with abase and, optionally, subsequent filtration. The removal of thecarboxylic acids by distillation requires different temperaturesaccording to the carboxylic acids used and the vacuum applied, althoughthey should not exceed the preferred reaction temperatures of up to 300°C.

According to the invention, particularly preferred reaction mixturescontaining polyester polyols are those which contain a high percentageof monomeric polyester polyols, the monomers containing on a statisticalaverage more than 1 and preferably more than 1.5 free hydroxyl groupsand--adjacent the hydroxyl group--ester groups of the carboxylic acidinitially introduced. In addition, the polyester polyol mixtures used inaccordance with the invention contain dimeric, trimeric and/or highercondensates which are formed, for example, by epoxide ring opening withester polyols already present. It is possible by gel permeationchromatography (GPC) (standard: (poly)styrene), to show that thepolyester polyol mixtures used in accordance with the invention containup to 50%, preferably up to 45% and, more preferably, around 40% byvolume of

The reaction mixtures containing polyester polyols-preferably used haveHoppler viscosities (DIN 53 015) at 20° C. below 9,000 mPa.s, preferablybelow 6,000 mPa.s and, more preferably, in the range from 50 to 5,000mPa.s. These reaction mixtures containing polyester polyols can thus becast at room temperature (20° C. to 25° C.).

According to the invention, particularly preferred reaction mixturescontaining polyester polyols are those which have been ring-opened byring opening of epoxidized soybean oil epoxide (epoxide oxygen contentapproximately 6.8% by weight) with head fractionated fatty acid--a fattyacid mixture of C₈ acids and C₁₀ acids which has an acid value of 300 to400. The reaction mixtures containing polyester polyols thus producedhave Hoppler viscosities at 20° C. of the order of 4,000 mPa.s.

The reaction mixtures containing polyester polyols may either be usedindividually or in admixture with polybutadiene diols as polyolcomponent in the production of polyurethane casting resins. DE-PS 2 847383 describes mixtures of polyether/polyester polyols with polybutadienediol as polyol component in the production of polyurethane materials.However, the polyether/polyester polyols used there have a relativelylow equivalent rate of 350 so that relatively short-chain polyurethanematerials which are friable and can be disintegrated by hand areobtained using polybutadiene diol and the internal plasticizer, 2-octyldodecanol, and aromatic diisocyanates.

DE-OS 25 30 676 describes a process for the production of polyurethanematerials which may be used, for example, for electrical applications.In this process, polyester polyols, polyether polyols and a polyhydroxypolymer of a 1,3-diene hydrocarbon containing 4 to 12 carbon atoms areused as the polyol component in quantities of 5 to 40% by weight, basedon the total weight of the reactants. However, the only suitablepolyester polyols based on renewable raw materials which are mentionedare the diglycerides of hydroxyl-containing castor oil, tall oil,soybean oil, linseed oil, etc.

According to the invention, the reaction mixtures containing polyesterpolyols may be used individually or in admixture with polybutadiene diolas the polyol component; polybutadiene diol may even be the predominantcomponent. The polybutadiene diol is preferably used in quantities of 20to 80 % by weight and, more preferably, in quantities of 40 to 60 % byweight. The quantities in which the polybutadiene diol are added areprimarily determined by the required properties of the polyurethanecasting resin. On the one hand, additions of polybutadiene diols improvethe elastic properties (measured as the Shore A hardness at 27° C.) ofthe polyurethane materials. On the other hand, the polybutadiene diol,which in addition is expensive, has a high viscosity (approx. 9,000mPa.s according to Hoppler at 25° C.), so that the polybutadiene diolcontent should be as low as possible in the interests of favorableprocessing.

Hydroxyl-terminated polybutadiene diol having a average molecular weightof approximately 3,000 and a degree of polymerization of the order of 50is used in accordance with the invention. The terminal primary hydroxylgroups are mainly in an allylic arrangement. The predominantconfiguration of the preferred polybutadiene diol is a trans-1,4-isomerwhich is present in admixture with the other cis-1,4- andvinyl-1,2-structures. The functionality of the polybutadiene diol usedis preferably in the range from 2 to 3 and more preferably in the rangefrom 2.2 to 2.6. Polybutadiene diol is commercially available and ismarketed, for example, by the American firm Atlantic Richfield Companywhich, for product information, has published a general brochure onpolybutadiene diol in which the properties, configuration data andpossible reactions of, polybutadiene diols are described.

One particular advantage of the invention is that the described reactionmixtures containing polyester polyols can be mixed in any ratio with thepolybutadiene diols. For example, the viscosity of the polyol componentcan be determined very effectively through the ratio in which thereaction mixture containing polyester polyols is mixed with thepolybutadiene diol. Since the polybutadiene diol used in accordance withthe invention basically has a viscosity of the order of 9,000 mPa.s,polyol components having desired viscosity values can be obtained bycalculated additions of the reaction mixtures containing polyesterpolyols having certain viscosities which vary according to theepoxidized esters and ring-opening carboxylic acids.

According to the invention, the reaction mixtures containing polyesterpolyols are used either individually or optionally in admixture withpolybutadiene diols in the reaction with diisocyanates to polyurethanecasting systems. To this end, the polyester polyol component or themixture of polyester polyol components is mixed with the isocyanatecomponents in known manner.

According to the invention, the polyol component(s) is/are used in suchquantities that the ratio of hardener OH groups to isocyanate groups isin the range from 1:0.9 to 1:1.3 and preferably in the range from 1:1 to1:1.2. The isocyanates used may be any compounds containing at least twoterminal isocyanate groups which are typically used for the productionof polyurethane polymers. Suitable isocyanate components are anyaromatic and aliphatic diisocyanates such as, for example,1,5-naphthylene diisocyanate, 4,4'-diphenyl dimethyl methanediisocyanate, dialkyl and tetraalkyl diphenyl methane diisocyanate, theisomers of tolylene diisocyanate, optionally in admixture,1-methyl-2,4-diisocyanatocyclohexane, 1,6-diisocyanato-2,2,4trimethylhexane, 1,6-diisocyanato-2,4,4-trimethyl hexane, chlorinated andbrominated diisocyanates, phosphorus-containing diisocyanates,butane-1,4-diisocyanate, dicyclohexyl methane diisocyanate,cyclohexane-1,4-diisocyanate, dimer fatty acid diisocyanate, tetramethylcyclohexyl diisocyanate, isophorone diisocyanate and isomers.

According to the invention, aromatic isocyanates containing 2 to 4isocyanate groups are particularly preferred. Suitable isocyanates suchas these are both compounds which contain all the isocyanate groups onan aromatic ring or several aromatic rings conjugated with one anotherand also compounds which contain the isocyanate groups on several ringsattached to one another by alkylene groups, for example methylenegroups. 2,4-Tolylene diisocyanate and 4,4'-diphenyl methane diisocyanate("MDI polymer"), for example, are suitable. In other embodiments,mixtures of 4,4'-diphenyl methane diisocyanate with isocyanates ofhigher functionality, for example with substituted diphenyl methanediisocyanates, which contain another aromatic ring bearing isocyanategroups as substituent are used as the isocyanate component. Commerciallyavailable liquid crude mixtures of diphenyl methane diisocyanate ("MDIpolymer") , which still contain oligomeric polyphenylene polymethylenepolyisocyanate, are particularly preferred. Of these liquid crudemixtures, those which have an average functionality of 2 to 2.5isocyanate groups per molecule are preferably used.

In one particular embodiment of the present invention, the polyolcomponent(s) are used in the so-called one-shot process, i.e. thestarting materials are directly mixed with one another, generally withsimultaneous addition of such auxilaries as blowing agents, catalysts,foam stabilizers and flameproofing agents. Suitable catalysts are any ofthe catalysts known to the expert for the production of polyurethanes.Particularly suitable catalysts are tertiary amines differing widely instructure, such as diazabicyclooctane (Dabco), triethyl amine anddimethyl benzyl amine. Other suitable additives are known additives ofthe type typically used in the production of polyurethane materials,including for example fillers, pigments and/or so-called drying agents.According to the invention, the zeolite pastes known to the expert arepreferably used as drying agents.

The polyurethane resin mixtures obtained by the use in accordance withthe invention of the reaction mixtures containing polyester polyols,optionally in admixture with polybutadiene diols, are liquid and ofrelatively low viscosity. They may readily be cast into shape and may becured, optionally at slightly elevated temperatures.

Solid polyurethane materials having excellent hardness, strength anddimensional stability and also electronic volume resistances can beobtained by the use of the reaction mixture containing polyester polyolsin accordance with the invention. Accordingly, these polyurethanematerials are particularly suitable for applications in the field ofelectrical insulation. In addition, the new polyurethane materials areobtained from starting materials which emanate from native sources andmay be readily obtained therefrom in high yields by inexpensive methods.Polyurethane materials having a wide range of properties can be obtainedby variation of the reaction mixtures containing polyester polyols withpolybutadiene diols and the diisocyanates.

EXAMPLES A. Preparation of the reaction mixture containing polyesterpolyols Example 1

1225 g head fractionated fatty acid (60% C₈, 35% C₁₀, acid value 361.9),corresponding to 7.9 mol based on the acid value, were initiallyintroduced into the reaction vessel and heated with stirring to 150° C.1770 g soybean oil epoxide (epoxide oxygen content 6.784 by weight),corresponding to 7.5 mol based on the epoxide content, were added whilestirring with delay (60 minutes) so that the absolute content ofunreacted epoxide-groups in the reaction mixture did not exceed 1.6% byweight. After, the addition, the reaction temperature was slowlyincreased to 170° C. and the reaction mixture was kept at thattemperature until the residual epoxide oxygen content had fallen below0.15% by weight (2 hours) . The head fractionated fatty acid reacted(470 g) was distilled off in vacuo (below 10 Pas) at up to 200° C.

A reaction mixture containing polyester polyols was obtained in the formof a clear yellow liquid having the following characteristic data: OHvalue approx. 95, saponification value 235, acid value 1.5, iodine valueapprox. 3.5, H₂ O<0. it by weight, Hoppler viscosity at 25° C. approx.4,000 mPa.s.

Example 2

50 Parts by weight of the reaction mixture containing polyester polyolsprepared in accordance with Example 1 were thoroughly stirred with 50parts by weight of the polybutadiene diol R-45 HT, a product of AtochemDeutschland GmbH (of which the characteristic data are shown inComparison Example 2).

The mixture obtained had a Hoppler viscosity at 27° C. of 4,200 mPa.s.

Comparison Example 1

A commercially available castor oil having the following characteristicdata was used as the polyol component: OH value 161, saponificationvalue approx. 180, iodine value 86, H₂ O<0% by weight, Hoppler viscosityat 25° C. approx. 700 mPa.s.

Comparison Example 2

A commercially available polybutadiene diol (R-45 HT, a product ofAtochem Deutschland GmbH) having the following characteristic data wasused as the polyol component: OH value 46.6, iodine value approx. 400,molecular weight approx. 2,800, Hoppler viscosity at 25° C. approx.9,000 mPa.s, H₂ O<0.1% by weight.

Example 3

The polyol components described in Examples 1 and 2 and in comparisonExamples 1 and 2 were mixed in the quantities shown in Table 1 (parts byweight) with the quantities--also shown in Table 1 --of diphenyl methanediisocyanate mixture (MDI polymer) containing 30 to 33% by weight NCO ina standard mixer, a drying agent (zeolite paste) being introduced asadditive in the quantitites (parts by weight) shown in Table 1. Nocatalyst was used in the production of this casting resin.

The properties of the casting resin systems obtained are also shown inTable 1.

                  TABLE 1                                                         ______________________________________                                                                  Comp.     Comp.                                              Ex. 1   Ex. 2    Ex 1.     Ex 2                                      ______________________________________                                        % By weight                                                                              100       100      100     100                                     polyol com-                                                                   ponent                                                                        Hoppler viscos-                                                                          3100      4200     650     8100                                    ity at 27° C. in                                                       mPa · s                                                              % By weight                                                                              25        18       40      11                                      isocyanate                                                                    MDI polymer                                                                   30-33% by                                                                     NCO                                                                           Drying agent                                                                              5         5        5       5                                      zeolite paste                                                                 % by weight                                                                   Pot life   >3 h      Approx.  Approx. Approx.                                                      50 mins. 25 mins.                                                                              40 mins.                                Properties of the casting resin after reaction                                Shore A hardness                                                              (27° C.) after                                                          24 h      --        26       57      --                                       48 h      35        31       63      --                                       72 h      42        35       66      --                                      168 h      50        40       68      --                                      Volume resis-                                                                            2 · 10.sup.15                                                                  6 · 10.sup.15                                                                 3 · 10.sup.14                                                                4 · 10.sup.16                  tivity after                                                                  168 h Ohm · cm                                                       ______________________________________                                    

Example 4

The polyol components described in Examples 1 and 2 and in ComparisonExamples 1 and 2 were mixed in the quantities shown in Table 2 (parts byweight) with the quantities--also shown in Table 2--of diphenyl methanediisocyanate mixture (MDI polymer) containing 30 to 33% by weight NCO ina standard mixer, a drying agent (zeolite paste) being introduced asadditive in the quantities (parts by weight) shown in Table 2. Incontrast to Example 3, diazabicyclooctane in the quantities shown inTable 2 was added as catalyst during mixing of the components. Theproperties of the casting resin systems obtained are also shown in Table2.

                  TABLE 2                                                         ______________________________________                                                                  Comp.     Comp.                                              Ex. 1   Ex. 2    Ex 1.     Ex 2                                      ______________________________________                                        % By weight                                                                              100       100      100     100                                     polyol com-                                                                   ponent                                                                        Hoppler viscos-                                                                          3100      4200     650     8100                                    ity at 27° C. in                                                       mPa · s                                                              % By weight                                                                              25        18       40      11                                      isocyanate                                                                    MDI polymer                                                                   30-33% by                                                                     NCO                                                                           Drying agent                                                                              5         5        5       5                                      zeolite paste                                                                 % by weight                                                                   Catalyst (Dabco)                                                                         0.5       0.5      0.5     0.5                                     % by weight                                                                   Pot life   10 mins.  7 mins.  9 mins. 5 mins.                                 Properties of the casting resin after reaction                                Shore A hardness                                                              (27° C.) after                                                          24 h      56        47       60      27                                       48 h      58        48       64      30                                       72 h      61        49       66      32                                      168 h      61        51       68      33                                      Volume resis-                                                                            4.5 · 10.sup.15                                                                1.6 · 10.sup.15                                                               1.3 · 10.sup.14                                                              4 · 10.sup.16                  tivity after                                                                  168 h Ohm · cm                                                       Tensile strength                                                                         1.4       0.6      1.4     0.18                                    MPa                                                                           Elongation %                                                                             38.8      38.8     30.6    79.5                                    ______________________________________                                    

We claim:
 1. The process of producing a polyurethane material which canbe cast at room temperature, comprising mixing a compound containing atleast two isocyanate groups per molecule in the presence of apolymerization catalyst with a reaction mixture containing a polyesterpolyol, said polyester polyol having been prepared by at least partialring opening of an epoxidized ester with a carboxylic acid wherein saidepoxidized ester has been added slowly to said carboxylic acid.
 2. Aprocess as in claim 1 wherein said polyester polyol contains on averagemore than 1 free hydroxyl group and ester group per molecule.
 3. Aprocess as in claim 1 wherein said polyester polyol comprises an esterof an epoxidized carboxylic acid with a monohydric to trihydric alcoholcontaining up to 40 carbon atoms.
 4. A process as in claim 1 whereinsaid epoxidized ester has been added slowly to said carboxylic acid toavoid substantial amounts of unreacted epoxide groups in said reactionmixture.
 5. A process as in claim 1 wherein said carboxylic acid hasbeen heated to a temperature of between above 80° C. and below 300° C.prior to addition of said epoxidized ester to said carboxylic acid.
 6. Aprocess as in claim 5 wherein said mixture has been reacted until anabsolute residual epoxide oxygen content in said reaction mixture ofless than about 1.0% by weight is obtained.
 7. A process as in claim 1wherein said reaction mixture has a Hoeppler viscosity at about 20° C.of below 9,000 mPa.s.
 8. A process as in claim 1 wherein said polyesterpolyol has been produced by ring opening of an epoxidized estercontaining more than one epoxide group.
 9. A process as in claim 1wherein said polyester polyol has been produced by ring opening of anepoxidized triglyceride.
 10. A process as in claim 1 wherein saidreaction mixture further contains a polybutadiene diol.
 11. A process asin claim 10 wherein said polybutadiene diol is present in the amount offrom about 20 to about 80% by weight, based on the weight of saidreaction mixture.
 12. A process as in claim 1 wherein said reactionmixture is present in such amount that the ratio of OH groups toisocyanate groups is from about 1:0.9 to about 1:1.3.
 13. A reactionmixture for producing a polyurethane material by casting, said reactionmixture comprising a polyester polyol prepared by at least partial ringopening of an epoxidized ester with a carboxylic acid wherein saidepoxidized ester has been added slowly to said carboxylic acid.
 14. Areaction mixture as in claim 13 wherein said polyester polyol containson average more than 1 free hydroxyl group and ester group per molecule.15. A reaction mixture as in claim 13 wherein said polyester polyolcomprises an ester of an epoxidized carboxylic acid with a monohydric totrihydric alcohol containing up to 40 carbon atoms.
 16. A reactionmixture as in claim 13 wherein said epoxidized ester has been addedslowly to said carboxylic acid to avoid substantial amounts of unreactedepoxide groups in said reaction mixture.
 17. A reaction mixture as inclaim 13 wherein said carboxylic acid has been heated to a temperatureof between above 80° C. and below 300° C. prior to addition of saidepoxidized ester to said carboxylic acid.
 18. A reaction mixture as inclaim 17 wherein said mixture has been reacted until an absoluteresidual epoxide oxygen content in said reaction mixture of less thanabout 1.0% by weight is obtained.
 19. A reaction mixture as in claim 13wherein said reaction mixture has a Hoeppler viscosity at about 20° C.of below 9,000 mPa.s.
 20. A reaction mixture as in claim 13 wherein saidpolyester polyol has been produced by ring opening of an epoxidizedester containing more than one epoxide group.
 21. A reaction mixture asin claim 13 wherein said polyester polyol has been produced by ringopening of an epoxidized triglyceride.
 22. A reaction mixture as inclaim 13 wherein said reaction mixture further contains a polybutadienediol.
 23. A reaction mixture as in claim 21 wherein said polybutadienediol is present in the amount of from about 20 to about 80% by weight,based on the weight of said reaction mixture.